Conventionally, it is necessary from the viewpoint of security to conduct person identification at the time of, for example, entering and exiting a room in a building, accessing a computer, and reception at a banking organ.
In a conventional fingerprint authentication apparatus which conducts the person identification, a fingerprint image is acquired by using a plane sensor or a line-shaped sensor using optical means, capacitance means or thermal means. A monochrome variable density image of fingerprint upheaval lines obtained by taking in a fingerprint of a finger of a discrimination subject is compared with a registered image of a fingerprint registrant to determine whether the discrimination subject is the same as the fingerprint registrant.
In image processing in such a fingerprint authentication apparatus, feature extraction is conducted on the acquired fingerprint image and collation is conducted by using an extracted feature quantity. In general, preprocessing such as noise removal and background removal is conducted on the fingerprint image before conducting the feature extraction, in order to extract a feature of the fingerprint with higher precision. The preprocessing is conducted with the object of preventing the collation precision from being lowered by false detection or a detection omission of the fingerprint feature and eliminating unnecessary processing to reduce the amount of calculation. For conducting the background removal included in the preprocessing, it is necessary to separate the subject fingerprint image into impressing regions where the fingerprint is present and background regions.
In most conventional methods of separating the fingerprint into the impressing regions and the background regions, threshold processing is conducted simply on pixel values of the fingerprint image to discriminate between the impressing regions and the background regions on the assumption that there is a significant difference in pixel value (density value) between upheaval line pixels and pixels other than upheaval lines in an input image supplied from a fingerprint input apparatus.
For example, in a method described in Patent Document 1 listed below, a high density part is transformed into a black region of pixel 1 and a low density part is transformed into a white region of pixel 0 by utilizing a density value of the input image, gradating the input image by using the Gaussian transform and temporarily converting the input image to a binary value. As a result, the black regions are discriminated as a fingerprint image part. Therefore, this part is masked and returned to the input detected image, and the white regions are cut as background information. The fingerprint image subjected to the background separation processing is subject to feature extraction processing. The background separation processing is premised on the assumption that there is a significant difference in pixel value distribution between the impressing regions and the background regions.
A method described in Patent Document 2 is not intended for a fingerprint but intended for a palm print. The impressing regions are found by extracting contour of the palm print. As for the contour of the palm print, a boundary is determined on the basis of a luminance difference between the background pixels and the palm print pixels by using scanning lines of the palm pixel image from downward, upward, rightward and leftward directions. Discrimination between upheaval line pixels and other pixels is conducted by simple threshold processing. This method is also premised on the assumption that there is a distinct difference in distribution between pixel values of upheaval lines and pixel values of the background and valley lines.
On the other hand, a method described in Patent Document 3 solves drawbacks of a method of fractionalizing a fingerprint image into small regions (blocks), utilizing the fact that there is a difference in pixel value distribution between the impressing regions and the background regions, and comparing pixel values in the small regions. In other words, proceeding to collation processing is conducted after converting the fingerprint image into binary values by using, for example, a method of using a middle value of pixel values, determining whether a fingerprint exists in the small regions obtained by the fractionalizing on the basis of continuity of upheaval lines and valley lines, and confirming that the fingerprint image is an image which does not include the background image and which is suitable for authentication.
FIG. 8 is a diagram showing one region to explain the method described in Patent Document 3.
A plurality of pixels (for example, 10 by 10 pixels) are included in one region. When a predetermined number of pixels 1 or 0 are consecutive along a decision line, the line is judged to be an upheaval line or a valley line.
Patent Document 1: Japanese Patent Application Laid-open No. 2003-44856
Patent Document 2: Japanese Patent Application Laid-open No. 10-255051
Patent Document 3: Japanese Patent Application Laid-open No. 2002-298126